This invention relates generally to radio frequency (RF) plasma generating devices, and more particularly, to transfer arc cleaning of a substrate during plasma processing of the substrate.
Of interest is copending application entitled "Elimination of Strike-over in RF Plasma Guns" filed Nov. 7, 1988, Ser. No. 267,865 by G. Frind and assigned to the assignee of the present invention.
Radio frequency (RF) plasma deposition is a plasma spray process which is well known for producing high temperature gaseous plasma. The devices for generating the plasma are sometimes referred to as plasma guns. They find utility in diverse heating applications such as high temperature chemical reactions, heating of solid targets, melting of particles such as a superalloy and for providing surface coatings and spray processes. Plasma processes are also used to produce low interstitial content titanium, refractory metal, as well as the superalloy deposits. In addition, the deposition efficiency of materials sprayed by the RF plasma process can approach 100%.
One major problem with RF plasma processing has been the presence of oxides on the surface of the substrate to be plasma sprayed. Oxide free surfaces are necessary to obtain strong bonding between the spray deposit and the substrate, since many oxides can prevent the interdiffusion of the deposit in the substrate in a subsequent heat treatment. It is the interdiffusion which is required to form a good metallurgical bond.
One kind of plasma deposition system employs DC current. In DC plasma systems, a process referred to as transfer arc cleaning allows the removal of oxides on the substrate to be plasma sprayed. In FIG. 1, a prior art DC plasma deposition system employing transferred arc cleaning of a substrate is shown. The DC plasma gun includes a water cooled copper anode 10 including a powder inlet 12 in which the particles to be deposited are injected. A tungsten cathode 14 is positioned adjacent to the anode 10. A DC power supply 16 provides a DC voltage between the anode 10 and the cathode 14 with the anode 10 at a positive potential. A second DC voltage supply 18 is coupled between the anode 10 and the substrate 20 which receives the deposited material. The substrate 20 is at a negative potential relative to the anode 10.
Power supply 16 produces an arc 22 from the cathode 14 to the anode 10. This produces a plasma 24 which flows from the cavity 26 formed by the anode 10 to the substrate 20. The plasma plume 24 is created by gases flowing in the cavity 26 of the anode 10. Arcs produced by the power supply 18 are transferred from the anode 10 to the electrically conductive substrate 20 using the exited plasma plume 24 as one leg of the conduction path. This transferred arc cleans the surface of the substrate 20 removing surface oxides from the substrate by the action of multiple arcs moving across the surface of the substrate.
The problem with RF plasma guns is that they lack transferred arc cleaning capability. The reason for this is that unlike the DC plasma system of FIG. 1 in which the system employs anode and cathode electrodes to which a power supply can be conveniently connected for producing a transferred substrate cleaning arc, typical RF guns do not have comparable electrodes. In an RF plasma gun, the plasma is produced by induced RF energy which causes the flowing gases to create a plasma plume which flows to the adjacent substrate. The present inventors recognize a need for providing a transferred arc cleaning capability to typical RF plasma guns.
An RF plasma system for transfer arc cleaning of substrate during plasma processing of a substrate in accordance with one embodiment of the present invention includes an RF plasma device. The RF plasma device includes an enclosure defining a chamber for containing a plasma and having a plasma exit port through which the plasma flows. An electrical induction coil is adjacent to the enclosure for applying RF energy to a region within the chamber to create a plasma from the gas flowing in the chamber. The device includes an electrically conductive member at the exit port and secured to the enclosure. The member is sufficiently close to the contained plasma to form an electrically conductive path therewith. A substrate supports holds a substrate adjacent to the port for processing by the plasma. Means are included for providing a direct current voltage between the member and the substrate support. The voltage has a polarity so that an electrical arc flows from the member to the substrate in response to the voltage.